There are several reason why your ad is often not shown on Google. There are certain updates, which often take place in the AdWords system and so also the reasons why this might happen. Let me summarize a few of the errors that results in non-display of your ads.

• First and foremost, as long as the billing data is not submitted and also the account has not been activated, the ads delivery will not start. At times, though the adwords account has been already activated, the ads may not be seen as they are still in the ‘Review Process’. So the ads will appear only after they have been reviewed as well as approved by the relevantAdWords specialist.
• If the keyword required minimum bid in a specific case happens to be higher than the actual bid set, the key term will get inactive and thus not show up in search results. Also, the ad will not be displayed for the rest of the given day, if the daily budget is already depleted.
• If the recommended budget has turbed higher than the set budget limit, the ads might not appear all the time. If the daily budget has got used up in the beginning of the day itself owing to accelerated method and you’re searching later in the day, you may not be able to see your ad.
• If the Ads are disapproved (let us assume that there are 2 ads in a particular Ad Group and both have been disapproved for not meeting the AdWords editorial guidelines), when the key terms that belong to that particular Ad Group are looked for, those ads won’t be shown.
• If you have chosen only Content network and are searching in Google, you won’t be able to see your ad. In case the Ad Scheduling feature is ON, the ad may not be shown for the time may perhaps be outside the scheduled time-slot. In case the Position Preference is probably enabled in the ‘Edit Campaign’ Settings, the ads might not be visible provided the bid is not high enough for them to be placed in the position we have selected. If you’re searching outside the targeted geo graphical settings, that ad will not be available.
• Campaign and adgroup should be active for ads to show up online. If someone pauses the ads/ keywords/ campaigns by accident, the ads may not appear. As an instance, if the campaign was scheduled to run only till a particular day, and if you’re searching on the next day, your ads won’t be there online.
• Certain keywords trigger Google’s red flag. The content policy gives you an idea of keywords that are taboo. Sometimes negative keywords tend to overlap with the searched keyword. Google then will not feature your ad.  Check for negative keywords and remove them from the campaign.